1. Field of the Invention
The present invention relates to an active matrix substrate used in, for example, a liquid crystal display apparatus. In particular, the present invention relates to an active matrix substrate using two-terminal nonlinear devices as switching elements.
2. Description of the Related Art
A liquid crystal display apparatus which is one kind of display for a user interface has widely been used in recent years, because it is superior to a CRT (Cathode Ray Tube) in display quality. Further, the liquid crystal display apparatus has advantages of saving space, light weight, low power, long life, and the like. Such apparatus have various applications, for instance in the field of office automation, and in an audio visual field. In particular, in order to realize a larger-size display with a high resolution, more improvement of the display quality is required. Therefore, the demand for a liquid crystal display apparatus of an active matrix driving type (hereinafter, referred to as "an active matrix display apparatus") has greatly been increased.
Active matrix display apparatus are roughly classified in two types according to their types of switching element. One uses three-terminal nonlinear devices such as TFTs (Thin Film Transistors) and the other uses two-terminal nonlinear devices. In the fabrication of the liquid crystal display apparatus using three-terminal nonlinear devices, a process for depositing a thin film and a photolithographic process for patterning the thin film require 6 to 8 or more times. This makes the fabrication of the liquid crystal display complicated, which raises the cost therefor. Thus, decreasing the cost is the most important problem. On the other hand, a liquid crystal display apparatus using two-terminal nonlinear devices as switching elements is superior to the liquid crystal display device using the three-terminal nonlinear devices for its lower cost. Accordingly, the liquid crystal display apparatus using the two-terminal nonlinear devices has rapidly been developed.
As a representative example of the two-terminal nonlinear device, a Metal-Insulator-Metal device (hereinafter, referred to as "an MIM device") is known. A liquid crystal display apparatus using the MIM devices as switching elements comprises an active matrix substrate on which pixel electrodes and MIM devices are formed, a counter substrate on which counter electrodes are formed, and a liquid crystal layer interposed therebetween. Steepness in the difference of the transmittance of the liquid crystal layer with respect to the change of the voltage applied to the liquid crystal layer is improved, so that a display with a high contrast can be obtained even in high duty driving in accordance with the larger-size display screen with a high resolution of the liquid crystal display apparatus.
A structure of the conventional active matrix substrate on which the MIM devices are formed as switching elements of a liquid crystal display apparatus will be described with reference to FIGS. 8 and 9. These figures show the active matrix substrate for a single pixel.
The active matrix substrate of FIG. 8 comprises a glass substrate 13. Signal lines 9 of Ta and lower electrodes 10 each branched from the signal lines 9 are formed on the glass substrate 13. An insulator 11 of Ta.sub.2 O.sub.5 is formed over each lower electrode 10. On each insulator 11, an upper electrode 12 of Ti is formed. An MIM device 7 comprises one lower electrode 10, one upper electrode 12, and one insulator 11 interposed therebetween. The upper electrode 12 is electrically connected to a corresponding one of pixel electrodes 8. The current flows through the signal line 9, the lower electrode 10, the insulator 11, the upper electrode 12, and the pixel electrode 8 in this order. Alternatively, the current can flow in the reverse order. Wiring formed of ITO or the like are formed on the counter substrate so as to cross the signal lines 9 to each other at a right angle. The active matrix substrate and the counter substrate are attached to each other so that surfaces thereof with the wirings face each other, constituting a liquid crystal cell.
The active matrix substrate of FIGS. 8 and 9 is fabricated as follows:
First, a thin Ta film is deposited on the glass substrate 13 so as to have a thickness of 3000 angstroms and then patterned into a desired shape by photolithography to form the signal lines 9 and the lower electrodes 10. Successively, an exposed portion of each lower electrodes 10 is anodized to form a Ta.sub.2 O.sub.5 film with a thickness of 600 angstroms thereon. The Ta.sub.2 O.sub.5 films work as the insulators 11. After that, a Ti film is deposited over the glass substrate 13 by sputtering or the like so as to have a thickness of 4000 angstroms and then patterned into a desired shape by photolithography to form the upper electrodes 12. Moreover, a transparent conductive film of ITO is deposited on the resulting glass substrate 13 so as to have a thickness of 1000 angstroms and then patterned to form the pixel electrodes 8.
It is preferred that the MIM device may have a symmetrical curve of a current-voltage characteristic between a state where the current flows from the lower electrode to the upper electrode and a state where the current flows from the upper electrode to the lower electrode. In order to realize such a symmetrical current-voltage characteristic, the lower electrode and the upper electrode may be formed of the same material. However, when the upper electrode is formed of the same material as that of the lower electrode, the lower electrode and the insulator obtained by anodization of the exposed portion of the lower electrode may probably be etched while the Ti film is patterned by photolithography to form the upper electrode. Due to this, in the case where the lower electrode and the upper electrode are formed of the same material, the patterning for forming the upper electrode cannot be conducted by photolithography. In view of such facts, material for the upper electrode is selected so that an anodized oxide film and the lower electrode are not etched.
Further, the material for the upper electrode thus selected should be selected so as not to damage the above-mentioned symmetrical current-voltage characteristic of the MIM device. For example, in the case where a Ta film is used for the lower electrode, a Ti, Al film, or the like can be used for the upper electrode.
In general, as shown in FIGS. 8 and 9, the upper electrode is formed before the formation of the pixel electrode. However, in the case of using an Al film for the upper electrode, the Al film is etched by an etchant for ITO while patterning of the ITO film. Therefore, the pixel electrode should be first formed as shown in FIGS. 10 and 11, and then the upper electrode should be formed so as to finally cover part of the pixel electrode.
In the liquid crystal display apparatus, a voltage applied to the MIM device as the switching element is divided to a capacity of the MIM device and a capacity of a liquid crystal layer interposed between the active matrix substrate and the counter substrate. In order to obtain a display with a high resolution by driving the liquid crystal layer, the capacity of the MIM device will be set so that the capacity of the MIM device is one-tenth or less of the capacity of the liquid crystal layer. For example, in the case where pixels of the liquid crystal display apparatus are formed at 300 .mu.m pitch, the MIM device has a size of approximately 5 .mu.m.times.6 .mu.m.
In the active matrix substrate shown in FIGS. 8 and 9, the current flows thought the signal line 9, the lower electrode 10, the insulator 11, the upper electrode 12, and the pixel electrode 8 in this order, or in the reverse order as mentioned above. However, in a case where the upper electrode is formed of Ti, an electrical barrier occurs in the vicinity of an interface between the pixel electrode 8 of an oxide (ITO or the like) and the upper electrode 12 of Ti, so that the pixel electrode 8 and the upper electrode 12 are not electrically well connected to each other. That is, the pixel electrode 8 and the upper electrode 12 are in non-ohmic contact with each other. Further, in the case of using a metal easily oxidizable such as Al as a material for the upper electrode 12, since the upper electrode 12 and the pixel electrode 8 of an oxide film such as an ITO film are directly in contact with each other, the Al or the like is oxidized on an interface between the upper electrode 12 and the pixel electrode 8 to form an oxide. As a result, the upper electrode 12 and the pixel electrode 8 are not electrically well connected to each other. Moreover, a voltage drop occurs in the vicinity of the interface between the pixel electrode 8 and the upper electrode 12, so that the symmetrical curve of the current-voltage characteristic of the MIM device is damaged. These cause undesirable phenomena such as the occurrence of a residual image and a flicker on the display of the liquid crystal display apparatus.